The design of various separatory techniques has become necessary in order to provide more specific and more efficient separations of hydrocarbons produced in less desirable or less naturally producing hydrocarbon reservoirs. With the continual diminishment of hydrocarbon reserves, the industry has gone to the use of enhanced oil recovery techniques. These enhanced techniques include such methods as subjecting low pressure petroleum reservoirs to nitrogen or carbon dioxide pressurized gas injection and oxygen fire flooding. When using a pressurizing medium or a miscibility enhancing meduim, the produced petroleum contains varying amounts of the medium used for production or pressure maintenance purposes. It is desirable to separate the injected medium from the produced petroleum and hydrocarbons. Typically, the high molecular weight liquid petroleum is easily separated from the injected medium, but the lighter hydrocarbons are difficult to separate from the injection medium. Particularly, C.sub.1 -C.sub.7 range hydrocarbons are generally hard to dissociate from the injection medium, such as carbon dioxide. Carbon dioxide is extremely difficult to separate from the very low molecular weight hydrocarbons such as methane, ethane and propane. Additionally, carbon dioxide forms an azeotrope with ethane which makes separation even more difficult for these components. The separation of methane from carbon dioxide is hampered by carbon dioxide freezing problems due to the necessary refrigeration required for the separation of methane.
Various techniques have been developed for the separation of producing mediums, such as carbon dioxide and nitrogen from valuable paraffinic hydrocarbons, such as the C.sub.1 -C.sub.7 range hydrocarbons. Refrigerated distillation is the most evident technique for separating hydrocarbons from mediums such as carbon dioxide. Refrigeration is used to provide a reflux in a distillation column to enhance the separation of the feed gas into a carbon dioxide overhead and a heavy hydrocarbon bottom stream. This technique suffers from the requirement of permissible carbon dioxide loss in the hydrocarbon product and high energy use as the amount of hydrocarbon recovery is raised to a high level.
Another technique designed to overcome the problem of hydrocarbon separation is activated methyldiethanolamine (MDEA). This process uses solvent in a two column scheme to separate CO.sub.2 as the bulk component from a produced hydrocarbon gas. The carbon dioxide is then recovered at low pressures, while the hydrocarbon gas remains pressurized. This process is very energy intensive because the carbon dioxide has to be recompressed to high pressures. Capital cost is also increased because of the necessity of a carbon dioxide compressor for enhanced recovery applications in which the carbon dioxide is reinjected into the producing formation.
It is also known to use a membrane process which takes advantage of the different rates of permeation between carbon dioxide and produced hydrocarbons to effect separation. Here again, while the feed is pressurized, the carbon dioxide is recovered as a low pressure product and has to be recompressed, making the process energy intensive. The addition of a compressor to the membrane process also makes the total process capital intensive.
Finally, various extractive distillation techniques are known and are generally referred to as Ryan-Holmes. This Ryan-Holmes technique is exemplified by U.S. Pat. No. 4,293,322, U.S. Pat. No. 4,318,723, U.S. Pat. No. 4,383,842, and U.S. Pat. No. 4,428,759. The latter patent discloses the use of extractive distillation using a C.sub.3 -C.sub.6 extractive solvent in a single column distillation process. Carbon dioxide is separated from various hydrocarbons and inerts, as well as hydrogen sulfide. In FIG. 3 of that patent, an extractive separation technique is set forth wherein carbon dioxide and ethane is separated from propane and higher hydrocarbons. The single extractive distillation column operates with a reboiler and condenser and the distillation column is supplied with solvent from a stripping column wherein the solvent is passed directly to the overhead or condenser of the distillation column without thermodynamic interaction of the various separation stages.
In contrast to the use of lower alkyl hydrocarbons as an extractant medium, such as in the Ryan-Holmes process, in USSR 975041 of Nov. 23, 1982, a technique for separation of carbon dioxide from hydrocarbons is set forth using various siloxanes. The method is particularly useful for separations wherein carbon dioxide constitutes over 60% of the gas mixture being treated.
The present invention overcomes the problems of the prior art in making a separation of hydrocarbons coproduced with petroleum using inert production media, such as carbon dioxide and nitrogen by providing a technique which is energy efficient and capital efficient and yet still effects the difficult separations of such gas species as carbon dioxide and propane.